A backplane is a specialized type of circuit board typically used to connect signals between multiple circuit cards or modules (e.g. populated printed circuit boards). Each such circuit card or module occupies a slot of the backplane and is plugged edgewise into the backplane by way of a respective connector soldered, press-fitted or otherwise installed to the backplane. Different architectures for connecting the multiple cards exist to accomplish different types of communication, and permit different levels of data transfer bandwidth.
One backplane class implements parallel multi-drop connections to provide signal links between cards. This architecture requires a large number of slow speed signals connected in parallel to achieve high bandwidth data transfer between cards. Since this architecture is multi-drop in nature, cards can be installed in different slot locations on the multi-drop signal bus without loss of communication. The multi-drop connection scheme has limited data bandwidth caused by all the cards being connected on the same signal lines.
Another backplane class implements point-to-point connections between cards for signal connections. This architecture usually implements very high speed signals transmitted over tuned copper circuit links. These signals, due to their high frequency nature, cannot be connected to multiple cards as is done in the multi-drop architecture. Because of the point-to-point nature of these connections, cards must be installed in specific slots to communicate.
Another class of backplane architecture implements both multi-drop and point-to-point signal connection types. This architecture also requires the circuit cards to be installed in specific locations to permit communication over the point-to-point high frequency links.
Each of these backplane architectures has a fixed set of connection link features. Known backplane architectures have trade-offs that limit the flexibility of card position on the backplane and limit high bandwidth data links between cards.
For example, while the multi-drop backplane architecture does allow the flexibility of connecting cards in different slot locations, multi-drop backplanes need a very large quantity of parallel, low frequency connections to achieve a high data link bandwidth between cards. Connectors used in this architecture frequently do not have a sufficient number of pins to achieve a high data bandwidth capability.
The point-to-point backplane architecture has a differing set of trade-offs. In order to achieve high data bandwidth, point-to-point backplane architectures require specialized high frequency tuned circuit signal links. When using this type of point-to-point signal connections, plug in cards are confined to fixed and specific locations on the backplane printed circuit board (PCB) to achieve connectivity, which may not be optimal for other mechanical and system implementation reasons. In addition, the high frequency, point-to-point connections can also be problematic when implemented directly in a backplane, due to noise impacts on signal quality from adjacent signal traces and power layers in the backplane.
When high frequency, point-to-point connections are made in a backplane, other situations arise from the signal path in the backplane that affect the tuned circuit characteristics. If the signal passes between layers through “via” style connections to complete a signal path, high frequency signals experience degradation from reflections caused by “stubs” in the signal path. Stubs are parts of the signal path that branch off the main path between the 2 connection points. The length of these stubs in the signal path determines the range of frequencies that are degraded. Efforts to remove these unwanted stubs from the signal path increase the cost of the backplane fabrication, by requiring the additional process of back drilling the via connections to eliminate these signal degrading stubs, caused by the PCB fabrication method.